The present invention relates to a method of preparing anhydrous sodium sulfide by drying hydrated sodium sulfide.
One can find a hydrated sodium sulfide with an Na2S fraction of, in particular, 60 to 62%, as a product available on the market. According to stoichiometric calculation, this corresponds approximately to Na2S.3H2O. By introducing hydrogen sulfide into concentrated sodium hydroxide, a sodium sulfide hydrate melt, with a content of about 60% to 62% Na2S, is formed at temperatures above 100xc2x0 C. This is placed on a cooling conveyor, on which it solidifies (T less than 90xc2x0 C.). After comminution, one generally obtains a scaly product, which is commonly available on the market.
From the state of the art, methods for the preparation of anhydrous sodium sulfide from Na2S.9H2O, Na2S.6 H2O or Na2S.5H2O are already known. In accordance with EP-B-0345 136, which is incorporated herein by reference, the monohydrate is prepared in a first dehydration stage. A time of 2 hours or more is necessary for the removal of water. The remaining water contained in the monohydrate is then removed at a temperature of 90 to 200xc2x0 C. and a reduced pressure of approximately 2.66 kPa (20 torr) to 66.5 kPa (500 torr). A similar two-stage method for the preparation of crystalline, anhydrous sodium sulfide is described in EP-B 0361 998, which is also incorporated by reference herein.
In the method described in EP 0924165, which is incorporated herein by reference, hydrated sodium sulfide, having a water content of 35 to 45 wt %, is heated under reduced pressure for the preparation of anhydrous sodium sulfide. The sodium sulfide is conveyed while it is being mixed by a device, the temperature of the supplied Na2S hydrate steadily rising from approximately 20xc2x0 C., at the point where the solid is fed, to xe2x89xa7180xc2x0 C. at the exit for the solid. A reduced pressure of  less than 20 torr is simultaneously maintained.
Disadvantages of these known methods include the fact that one must wait for the complete reaction, for example, to the monohydrate, in the first stage (EP-B-0345 136), before this compound can be supplied to the second temperature stage, or that the vaporization rate is very high (EP 0924165).
The object of the invention is therefore to provide a method by which an essentially anhydrous sodium sulfide is obtained, while avoiding the disadvantages inherent in existing processes.
This and other objects of the invention are obtained by a method for the preparation of anhydrous sodium sulfide with a content of at least 98 wt % Na2S, preferably at least 99 wt. % Na2S, by drying hydrous sodium sulfide, with a water content of 35 to 45%, wherein the hydrous sodium sulfide is used as a solid or in the liquified form, and is dried at a high temperature while it is being mixed and conveyed.